1. Field of the Invention
The present invention relates to a method of coding a picture signal with a high efficiency.
2. Description of the Prior Art
It is now known that the digital form of a picture signal can be transmitted at a speed of more than 100 Mbps and can hardly be carried cut through existing communication lines in the terms of transmittable amount and cost. Various picture coding methods and their apparatus have been introduced offering the techniques of eliminating the redundancy of picture signals and reducing the speed of signal transmission. One of them is an orthogonal transform coding apparatus which has widely been employed.
A prior art orthogonal transform apparatus will be described referring to the drawings.
It is understood that a natural image in which less movement is involved allows the horizontal and vertical and time base correlation between pixels to be higher and its corresponding transform coefficients are great in energy at the low sequence and small at the high sequence.
FIG. 8 is a block diagram showing a prior art 3-dimension orthogonal transform coding apparatus.
As shown in FIG. 8, there are provided an input buffer 81, an orthogonal transform block 82, a zonal sampling block 83, a 3-dimensional block activity estimation block 84, a 3-dimensional block classification block 85, a transform coefficient energy calculation block 86, a bit allocation calculation block 87, a normalization block 88, a transform coefficient quantization block 89, a multiplexer 810 and an output buffer 811.
An orthogonal transform coding method associated with the prior art orthogonal transform coding apparatus will is described below.
As shown in FIG. 8, the image data of K-frames are fed and stored in the input buffer 81. Then, they are read out in the form of M.times.M.times.K 3-dimensional blocks and orthogonally transformed by the orthogonal transform block 82. High sequence coefficients of the corresponding transform coefficients delivered from the orthogonal transform block 82 are blocked in three dimensions by the zonal sampling block 83 while the remaining low sequence coefficients are passed through. The transform coefficients passing the zonal sampling block 83 are transferred to the activity estimation block 84 where the activity in the image of the 3-dimensional block is calculated from: ##EQU1## where F(u, v, w) is a transform coefficient of the 3-dimensional block. The resultant activity values are then fed to the classification block 85 and separated into k-number of classes by the amount of energy of each 3-dimensional block. In each class of the 3-dimension blocks, the variance of the transform coefficient is calculated by the energy calculation block 86. From the resultant variance, the bit allocation in each class is then calculated by the bit allocation calculation block 87 for providing a bit map, using an equation expressed by: EQU N.sub.k (u,v,w)=1/2log.sub.2 [.sigma..sub.k.sup.2 (u,v,w)]-log.sub.2 [D] (u,v,w).noteq.(0,0,0)
where .sigma..sub.k.sup.2 (u, v, w) is a variance of the transform coefficient assigned to a k class and D is a parameter. Nk(u, v, w) can be determined for a desired speed of transmission by controlling the value of D. The bit allocation calculation block 87 delivers a coefficient for normalization in each class. The normalization coefficient may be a variance or the maximum value of a transform coefficient contained in the class. The transform coefficients from the orthogonal transform block 83 are also transferred to the normalization block 88 for normalization with the normalization coefficient and then, quantized by the quantizer 89 according to the bit allocation computed. The quantized output, the bit allocation map, and the class identifier are fed to the multiplexer 810 for coding and controlled in the transmission velocity by the output buffer 811 prior to delivery to the transmission line.
As described above, the high sequence coefficients associated with small energy and low visibility are removed by zonal sampling of the transform coefficients so that a signal compression with a reduced degradation of picture quality can be ensured. Also, the bit allocation corresponding to the energy of each block is made and thus, the compression will be possible considering the local properties of an image (for example, as disclosed in "Transform Coding of Images" by R. J. Clarke, issued by Academic Press in 1985).
However, according to the aforementioned arrangement, the high sequence coefficients are removed systematically and the picture quality of a particular image, e.g. a miniature image, which contains a large number of high sequence components will be reduced to a considerable degree. Also, the moving region of the image contains more time-base high sequence components of the 3-dimension orthogonal transform coefficients and less spatial high sequence components while the still image region contains less the former and more the latter. Accordingly, when the time-base high sequence components are removed, the active image will appear discontinuous in motion.
The bit allocation is determined by measuring the amount of energy not distinguishing between the moving and unmoving regions of the image and the bit allocation map will be established regardless of the distinct movements. As the result, the drawback is that the bit allocation is provided restricting the movement in the moving region and decreasing the resolution in the still region.